The data you seek do not exist for our little turbos, but for prop jets and turbine engines the evidence is ample. Look under "wet compression" of "fogging" or "overspray" or "swirl flash" and you'll find the papers and studies.
In addition we have the testimony (the word comes from if you lie, your testicles are lopped off) of HotRod and some others that you can improve the efficiency of compression itself.
The only advantages to injecting pre-turbo are 1) you want a bigger turbo but are not ready to role the dice, and 2) you have no intercooler, can't fit one, or don't want one, and 3) there may be a thermal advantage to not generating the heat early in the air flow (w/out heating the intake system).
To me number 1 is paramount the others. The quest for a greater dynamic range in turbos has been a long sought pursuit--variable vane turbos, twin turbos of differing size, larger turbos with less spool friction (i.e. ball bearing), external wastegates, and so on.
That said, water injection is still a marvelous means of increasing the density of the air, suppressing detonation, and permitting combustion with more power/push (leaner AFR), even if injectin post-turbo.
It's not the holy grail, but it's sure is fun thinking about, and pretty cool if you can make you 400 CFM compressor flow 500 CFM. Penny per HP it's a huge mod.
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